Biosensors are analytical devices that allow for the detection of one or more analytes or species using a biological constituent for molecular recognition in combination with a detector component. The detection aspect of biosensors may operate based on the transduction of, optical or electrochemical changes that occur in response to the presence or binding of the analyte or species with the biological constituent. Many biosensors require that a label be attached to a target analyte. The amount or some proxy of the amount of label is detected and assumed to correspond to the number of target molecules. Labels can include fluorophores, magnetic beads, enzymes that generate a detectable species, or the like. Labels, however, can be expensive and can increase the overall time of the detection process. Some approaches, such as electrical-based biosensors, have attempted to eliminate the use of labels. A variety of different electrical measurements can be used in electrical-based biosensors. For instance, there are voltammetric sensors, amperometric/coulometric sensors, and impedance sensors.
Phage-displayed peptide libraries have been investigated as a potential tool that could offer the ability to test or screen for a large number of target molecules. For example, phage-displayed peptide libraries having on the order of 1010 unique members offer the promise of universal biorecognition. Unfortunately, this technology has found only limited application in biosensors. In prior work, detecting molecular recognition between phage and target has focused on a “sandwich assay” scheme involving the detection of phage binding to immobilized target using rather complicated and expensive testing equipment.
In some instances, the molecular recognition elements are covalently bound or otherwise linked to the electrode surface. For example, U.S. Patent Application Publication No. 2009-0092965 describes a biosensor device that uses a self-assembled monolayer that is interposed between an electrode and the bound virus particles. Sometimes, however, the linking chemistry is not always reliable and the required surface functionalization processes can be time consuming.
Nanowires offer versatile and unique properties for use in chemical and biological sensing applications. The chemical sensitivity of nanowires typically results from their high surface-to-volume ratios. The nanometer-scale, in addition to providing capacity for high-density parallelization, is well suited for biological systems. Interest in nanowire-based sensors derives primarily from the potential for such sensors to be label- and reagent-free; direct electrical sensing with nanowires could ultimately deliver a real-time device with the attributes of small size, low cost, and potential for high-throughput measurements.
Conventional nanowire biosensors use semiconducting nanowires, surface-modified with receptors such as antibodies, in a field-effect transistor configuration. The binding of a charged analyte molecule to these receptors induces a conductivity change in the nanowire by coulombically accumulating or depleting charge carriers. Silicon and indium oxide semiconductor nanowires, for example, have been used to directly sense pH, metal ions, small molecules, proteins, lipoproteins, and DNA. Nanowires have also been used as the basis for biosensors that detect virus particles.
Conducting polymers, such as polythiophene, polypyrrole, and PEDOT, provide new opportunities for the incorporation of receptors into nanowires for biosensing. In contrast to metals and inorganic semiconductors, conducting polymers have a high degree of structural malleability and flexibility; additionally, conducting polymers offer some degree of porosity to potentially allow access to solvent and prospective analyte molecules. Nanoparticles, nanowires, and other sub-micron scale structures have been synthesized to allow tunable charge-transport, and offer a wide range of chemical and physical properties. For measurements in biological systems, the stability to physiological conditions of conducting polymer nanowires provides high intrinsic biocompatibility. Doped conducting polymers have inherent electrical conductivity resulting from the presence of charge carriers and the mobility of the carriers in a conjugated system. Conducting polymer-based sensors have been used to detect ammonia, chloroform, hydrogen, acetic acid, and other compounds.
The specificity and selectivity of conducting polymer biosensors, either in thin films or nanowires, can be customized with biomolecules providing molecular recognition. The syntheses of conducting polymers can be made compatible with the integration of biomolecules, which typically require aqueous conditions at moderate temperatures and neutral pH. Approaches to incorporate biomolecules into conducting polymers include attachment to a monomer before polymerization, entrapment during synthesis, or conjugation after synthesis. To date, immobilized recognition elements include metal ions, antibodies, DNA, proteins, and enzymes. Viruses, which offer versatile platforms for molecular recognition, have not been previously incorporated into conducting polymer nanowire-based biosensors.
M13, a bacteria infecting virus or bacteriophage, can recognize essentially any analyte by binding to engineered polypeptides displayed on its surface, which can be altered through manipulation of the phage-packaged DNA. The protein coat of the virus provides densely packed receptors for avidity-based binding to analytes. Thus, the receptors selected from phage-displayed libraries can bind to small molecules, proteins, DNA, and viruses. Inexpensive, readily produced, and available in large quantities, these viruses infect only their host E. coli bacteria. M13 viruses can also form films patterned by an underlying polymer, or template the synthesis of materials through binding to phage-displayed peptides. With proven capabilities for engineered molecular recognition and materials, viruses could provide new approaches to electrical conductivity-based biosensing.
Even if nanowires are desirable from a biosensor perspective, several technical hurdles need to be surmounted. For example, arbitrary patterns of conductive polymer nanowires need to be fabricated in a controlled yet high-throughput manner. Further, custom molecular recognition motifs need to be integrated into the nanowires. Further, the molecular recognition scaffolds need to tolerate the harsh conditions of fabrication.